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20)     x - 2y  = 18   ⇒  x = 18 + 2y

Sub the into the second equation for x    and we have

-6(18 + 2y) - 4y = 4      simplify

-108 - 12y - 4y = 4      

-108 - 16y = 4            add 108 to both sides

-16y = 112            divide both sides by -16

y  =  -7 

And

x  =  18 + 2(-7)   =   18 - 14    =    4

21) 

4x + 8y  = -16      (1)   

-8x + 10y = -20    divide through by 2  ⇒  -4x + 5y = -10     (2)

Add (1) and (2)    and we get that

13y = -26             divide both sides by 13

y = - 2

And using (2) to find x, we have

-4x + 5(-2) = -10

-4x  - 10 = - 10       add 10 to both sides

-4x = 0

x = 0